Controlling plasma arc torches and related systems and methods

ABSTRACT

In some aspects, methods for preserving a usable life of a plasma arc electrode consumable installed in a plasma arc torch can include measuring a characteristic of an electrical signal being provided to the torch to generate a plasma arc between the torch and a workpiece to be processed; monitoring the characteristic during operation of the torch over a time period; comparing the characteristic to a threshold value; and, responsive to determining that a measured characteristic meets and/or exceeds the threshold value, initiating an arc extinguishing sequence to preserve the life of the electrode.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/237,780 filed Oct. 6, 2015, entitled“Controlling Plasma Arc Torches and Related Systems and Methods,” thecontents of which are hereby incorporated herein by reference in theirentirety.

TECHNICAL FIELD

This disclosure relates generally to thermal cutting torches (e.g.,plasma arc torches), and more specifically to controlling plasma arctorches and related systems and methods.

BACKGROUND

Thermal processing torches, such as plasma arc torches, are widely usedin the heating, cutting, gouging, and marking of materials. A plasma arctorch generally includes an electrode, a nozzle having a central exitorifice mounted within a torch body, electrical connections, passagesfor cooling, and passages for arc control fluids (e.g., plasma gas). Aswirl ring can be used to control fluid flow patterns in the plasmachamber formed between the electrode and the nozzle. In some torches, aretaining cap can be used to maintain the nozzle and/or swirl ring inthe torch body. In operation, a plasma arc torch produces a plasma arc,which is a constricted jet of an ionized gas with high temperature andsufficient momentum to assist with removal of molten metal. Power usedto operate plasma arc torches can be controlled by a power supplyassembly of a plasma operating system. The power supply can include aplurality of electronic components configured to control and supply anoperational current to the plasma arc torch, the gas flows provided tothe plasma arc torch, and, in some cases, motion of the plasma arctorch.

During a cutting sequence, a plasma jet is used to first pierce througha workpiece to form an initial pilot hole. The pilot hole typically mustbe formed before the torch can be moved to form the cut. Conventionalsystems are typically unable to determine how long it actually takes forthe plasma jet to fully pierce a workpiece. Therefore, empirical data isused to determine a conservative time estimate in which worn consumablescould fully perform the piecing operation. As a result of theseconservative time estimates, plasma jets are commonly held in place formore time than needed to pierce the workpiece, which can result in lostprocessing time, unnecessary electrode wear, and potential workpiecewarping or damage.

Some systems are configured to implement certain gas or electricalparameters during start up or shut down based on desired characteristicsof the arc during use. For example, some systems, upon intentionalextinguishing of the plasma arc, can provide desirable gas flow orcurrent profile combinations for shut down.

Typically, after prolonged use, consumables, such as electrodes, canphysically degrade and begin to mechanically break down due to heat andpressure. In some cases, electrodes can undergo catastrophic failure,which can be referred to as a complete blowout in which molten metalportions of the consumable may flow back into the torch cloggingpassageways, unintended component arcing may occur, and/or the electrodecan break apart and be expelled into the torch. Such actions can causedamage to the torch.

SUMMARY

In some aspects, methods, and related systems and means for carrying outthe methods, for preserving a usable life of a plasma arc electrodeconsumable installed in a plasma arc torch, can include measuring acharacteristic of an electrical signal being provided to the torch togenerate a plasma arc between the torch and a workpiece to be processed;monitoring the characteristic during operation of the torch over a timeperiod; comparing the characteristic to a threshold value; andresponsive to determining that a measured characteristic meets and/orexceeds the threshold value, initiating an arc extinguishing sequence topreserve the life of the electrode.

Embodiments can include one or more of the following features.

In some embodiments, the characteristic represents a pulse widthmodulation duty cycle. In some embodiments, the threshold value isgreater than about 80%. In some embodiments, the method further includescomparing the bus voltage to an incoming voltage to determine a dutycycle percentage of at least one of a chopper or a pulse widthmodulator. In some embodiments, the characteristic comprises a pulsewidth modulation value. The characteristic can include a rate of changeof a pulse width modulation value. In some embodiments, the arcextinguishing sequence comprises causing a decrease in a plenum gaspressure. In some embodiments, the arc extinguishing sequence comprisescausing a decrease in arc current. In some embodiments, the arcextinguishing sequence is completed in less than about 70 milliseconds.In some embodiments, the measured characteristic meeting or exceedingthe threshold value indicates an increasing distance between the plasmaarc torch and an arc attachment point where the plasma arc attaches tothe workpiece. In some cases, the increasing distance indicates that thetorch has reached at least one of an edge of the workpiece. In somecases, the increasing distance indicates that the torch has reached akerf area. In some embodiments, the threshold is predefined for aparticular cutting process. In some embodiments, the comparing thecharacteristic comprises referencing a lookup table of threshold valuesfor a particular cutting process.

In some aspects, methods, and related systems and means for carrying outthe methods, of operating a plasma arc torch and extinguishing a plasmaarc during a material processing operation to limit electrode wearresulting from unintended arc loss can include initiating the plasma arcbetween an electrode of the plasma arc torch and a workpiece to beprocessed with the torch; translating the torch along the workpiece andperforming a processing operation to the workpiece; advancing the torchtowards a void defined by the workpiece; responsive to the torchreaching the void, detecting a lengthening of the plasma arc between thearc attachment point and the torch; and based on the detected plasma arclengthening, initiating an arc extinguishing sequence to limit wear ofthe electrode.

Embodiments can include one of more of the following features.

In some embodiments, the void comprises a peripheral edge of theworkpiece. In some embodiments, the detecting the lengthening of theplasma arc comprises monitoring a characteristic of an electrical signalbeing provided to the torch to generate the plasma arc and comparing thecharacteristic to a threshold value. In some cases, the characteristiccomprises measuring a bus voltage. In some cases, the methods furthercomprise comparing the bus voltage to an incoming voltage to determine aduty cycle percentage of at least one of a chopper or a pulse widthmodulator. In some embodiments, the characteristic comprises a pulsewidth modulation value. In some embodiments, the characteristiccomprises a rate of change of a pulse width modulation value. In someembodiments, the arc extinguishing sequence comprises causing a decreasein a plenum gas pressure. In some embodiments, the arc extinguishingsequence comprises causing a decrease in arc current. In someembodiments, the arc extinguishing sequence is completed in less thanabout 70 milliseconds.

In some aspects, plasma arc material processing systems can include atorch including a torch body configured to receive a set of consumablecomponents connected to the torch body and configured to generate aplasma arc; and a power supply operably connected to the torch, thepower supply comprising: a means for measuring a characteristic of anelectrical signal being provided to the torch to generate the plasma arcbetween the torch and a workpiece to be processed; a means formonitoring the characteristic during operation of the torch over a timeperiod; a means for comparing the characteristic to a threshold value;and a means for, responsive to determining that a measuredcharacteristic meets and/or exceeds the threshold value, initiating anarc extinguishing sequence to preserve the life of the electrode.

In some aspects, methods, and systems and means for carrying out themethods, for limiting damage to a plasma arc torch body resulting from aconsumable failure within the torch can include determining a specifiedconductivity parameter set point of a current to be provided to theplasma arc torch for a material processing operation; measuring adetected conductivity parameter of plasma arc current being provided tothe plasma torch to perform the material processing operation; comparingthe specified conductivity parameter set point to the detectedconductivity parameter of plasma arc current and calculating an errorterm signal; and based on a determination that the error term signalexceeds a threshold amount, initiating a plasma arc shut down sequenceto extinguish the plasma arc to limit damage to the plasma arc torchbody.

Embodiments can include one or more of the following features.

In some embodiments, the specified conductivity parameter set point of acurrent to be provided to the plasma arc torch comprises a plasma arccurrent set point. In some embodiments, the specified conductivityparameter set point of a current to be provided to the plasma arc torchcomprises a plasma arc voltage set point. In some embodiments, the errorterm signal comprises a compilation of multiple comparisons of thespecified current set point to the detected plasma arc current atmultiple times during a material processing operation. In some cases,the multiple times comprise a predetermined time interval. In somecases, the multiple comparisons comprises a rolling sum of multipleerror term signals detected over consecutive plasma arc currentmeasurements. In some cases, the multiple error term signals comprisesabout 5 error term signals to about 20 error term signals. In someembodiments, the method also includes referencing a lookup table ofexample error term signal threshold amounts. In some cases, the lookuptable comprises multiple example error term signal threshold amounts. Insome cases, each of the multiple example error term signal thresholdamounts correspond to different material processing scenarios. In someembodiments, an increasing error term indicates a physical degradationof the consumable in the plasma torch. In some embodiments, the plasmaarc shut down sequence comprises causing a decrease in a plenum gaspressure. In some embodiments, the plasma arc shut down sequencecomprises causing a decrease in arc current. In some embodiments, theplasma arc shut down sequence is completed in less than about 70milliseconds.

In some aspects, methods, and systems the means for carrying out themethods, of operating a plasma arc torch can include initiating a plasmaarc in the plasma arc torch and sending a signal to operate the plasmaarc at a defined operating current; detecting an actual current level ofthe plasma arc; determining a deviation between the defined operatingcurrent and the actual current level; responsive to determining that thedeviation is below a threshold error level, permitting the plasma torchto continue operating; after a predetermined sample cycle time,redetecting an actual current level of the plasma arc to determine anupdated deviation between the defined operating current and theredetected actual current level; and responsive to determining that theupdated deviation meets or exceeds the threshold error level, initiatinga plasma arc extinguishing sequence to extinguish the plasma arc tolimit damage to the plasma torch body.

Embodiments can include one or more of the following features.

In some embodiments, the method also includes a third or more detectionand comparison sequence of the actual current level of the plasma arc tothe defined operating current. In some embodiments, the updateddeviation comprises a rolling sum of deviations determined during a setof detection and comparison sequences. In some cases, the updateddeviation comprises a rolling sum of deviations determined during a setof about 5 to about 20 detection and comparison sequences. In somecases, the updated deviation comprises a rolling sum of deviationsdetermined during a set of about 10 detection and comparison sequences.In some embodiments, the method further includes referencing a lookuptable of example threshold error levels. In some cases, the lookup tablecomprises multiple threshold error levels. In some embodiments, the arcextinguishing sequence comprises causing a decrease in arc current. Insome embodiments, the arc extinguishing sequence is completed in lessthan about 70 milliseconds.

In some aspects, plasma arc material processing systems can include aplasma arc torch including a torch body configured to receive a set ofconsumable components connected to the torch body and configured togenerate a plasma arc; and a power supply operably connected to thetorch, the power supply comprising: a means for determining a specifiedconductivity parameter set point of a current to be provided to theplasma arc torch for a material processing operation; a means formeasuring a detected conductivity parameter of plasma arc current beingprovided to the plasma torch to perform the material processingoperation; a means for comparing the specified conductivity parameterset point to the detected conductivity parameter of plasma arc currentand calculating an error term signal; and a means for, based on adetermination that the error term signal exceeds a threshold amount,initiating a plasma arc shut down sequence to extinguish the plasma arcto limit damage to the plasma arc torch body.

In some aspects, methods, and systems and means for carrying out themethods, of initiating a plasma arc of a plasma arc torch to pierce aworkpiece and detecting plasma piercing through the workpiece to begin acutting sequence can include calculating a pulse width modulationcharacteristic of an electrical signal associated with a plasma arcbetween an electrode of a plasma arc torch and a workpiece to beprocessed; monitoring the characteristic during operation of the torchover a time period of a workpiece piercing sequence; comparing thecharacteristic to a threshold value; and responsive to determining thata calculated characteristic meets and/or exceeds the threshold value,ending the workpiece piercing sequence and beginning the cuttingsequence and causing the plasma arc torch to move relative to theworkpiece to form a cut.

Embodiments can include one or more of the following features.

In some embodiments, the characteristic comprises a rate of change of apulse width modulation control parameter. In some cases, thecharacteristic comprises a rate of change of a pulse width modulationduty cycle. In some embodiments, the comparing the characteristiccomprises referencing a lookup table of threshold values for aparticular cutting process. In some cases, the lookup table comprisesmultiple threshold values, each being associated with a set of cuttingparameters. In some embodiments, the methods also include, during thepiercing sequence, delivering a gas mixture comprising an inert gas as aplasma gas and/or a shield gas to surround the plasma, the presence ofthe inert gas reducing a maximum plasma arc voltage required to breakthrough the workpiece during the piercing sequence. In some embodiments,the beginning the cutting sequence comprises changing at least one of aplasma gas or a shield gas from piercing gas mixture comprising an inertgas to a different gas mixture. In some cases, the piercing gas mixturecomprising the inert gas is a plasma gas. In some cases, the piercinggas mixture comprising the inert gas is a shield gas and the differentgas mixture comprising air of oxygen for the cutting sequence. In someembodiments, the beginning the cutting sequence and causing the plasmaarc torch to move relative to the workpiece to form the cut comprisessending a signal, from the power supply to a gantry controller, to causerelative motion between the torch and the workpiece.

In some aspects, methods, and systems and means for carrying out themethods, of performing a piercing sequence to pierce a hole in aworkpiece with a plasma arc torch can include initiating an arc betweenthe electrode and a nozzle of the plasma arc torch during a pilot arcmode; transferring the arc from the nozzle to a workpiece for piercingand cutting the workpiece; beginning a workpiece piercing sequence;estimating a characteristic associated with a pulse width modulatedsignal being provided to the power module to maintain the arc betweenthe torch and the workpiece during the workpiece piercing sequence;comparing the characteristic to a predetermined value; and responsive todetermining that an estimated characteristic meets and/or exceeds thepredetermined value, initiating relative movement between the workpieceand the plasma arc torch to begin a cutting sequence.

Embodiments can include one or more of the following features.

In some embodiments, the characteristic comprises a pulse widthmodulation control parameter. In some cases, the characteristiccomprises a rate of change of a pulse width modulation duty cycle. Insome embodiments, the initiating relative movement between the workpieceand the plasma arc torch comprises sending a signal from a power supplyin communication with the plasma arc torch to a movement controllermechanically coupled to the plasma arc torch. In some embodiments, thedetermining that the estimated characteristic meets and/or exceeds thepredetermined value identifies that plasma from the plasma arc torch hasbroken through the workpiece. In some embodiments, the initiatingrelative movement between the workpiece and the plasma arc torch isdelayed by a predetermined time period. In some embodiments, delayingthe initiating relative movement opens a semi-conical pierced hole inthe workpiece into a substantially cylindrical pierced hole using plasmaexpelled from the plasma arc torch.

In some aspects, methods, and systems and means for carrying out themethods, for increasing a usable number of pierce operations for anelectrode of a plasma arc torch can include initiating an arc betweenthe electrode and a corresponding nozzle of the plasma arc torch duringa pilot arc mode; transferring the arc from the corresponding nozzle toa workpiece for piercing and cutting the workpiece; increasing a currentof the plasma arc; delivering a gas mixture comprising an inert gas asat least one of a plasma gas or a shield gas to surround the arc andinitiating a piercing sequence; responsive to determining that thepiercing sequence has completed, beginning a cutting sequence and movingthe plasma arc torch relative to the workpiece.

Embodiments can include one or more of the following features.

In some embodiments, the presence of the inert gas reduces a maximumplasma arc voltage required to break through the workpiece during thepiercing sequence. In some embodiments, the presence of the inert gasincreases an arc attachment time during the piercing sequence. In someembodiments, the presence of the inert gas increases a workpiecepiercing time. In some embodiments, the beginning a cutting sequencecomprises providing a motion signal to a CNC controller to initiaterelative motion between the torch and the workpiece. In someembodiments, the beginning a cutting sequence comprises delivering asecond gas mixture to surround the arc that is distinct from gas mixturedelivered during the piercing sequence. In some embodiments, the gasmixture comprises an inert gas as a plasma gas during the piercingsequence. In some embodiments, the gas mixture delivered during thepiercing sequence comprises a plasma gas comprising argon and a shieldgas comprising oxygen, and the beginning a cutting sequence comprisesdelivering a second gas mixture to surround the arc comprising a plasmagas comprising nitrogen and a shield gas comprising argon. In someembodiments, the gas mixture comprising the inert gas is delivered froma metering console located on or near the plasma arc torch. In someembodiments, the determining that the piercing sequence has completedcomprises: estimating and monitoring a characteristic of an electricalsignal associated with the arc between the electrode and the workpiece;comparing the characteristic to a threshold value; and determining thatan estimated characteristic meets and/or exceeds the threshold value. Insome cases, the characteristic comprises a pulse width modulationcontrol parameter or a rate of change of a pulse width modulationcontrol parameter.

In some aspects, methods, and systems and means for carrying out themethods, of operating a plasma arc torch can include operating theplasma arc torch in an arc initiation mode by generating an arc betweenan electrode and a nozzle in the presence of a first set of shield andplasma gases; operating the plasma arc torch in a transferred piercingmode by transferring the arc from the nozzle to a workpiece in thepresence of a second set of shield and/or plasma gases, at least one ofthe shield or plasma gases of the second set at least partiallycomprising an inert gas; and based on determining that the arc has fullypierced the workpiece, operating the plasma arc torch in a transferredcutting mode by moving the torch relative to the workpiece in thepresence of a third set of shield and plasma gases.

Embodiments can include one or more of the following features.

In some embodiments, the operating the plasma arc torch in a transferredcutting mode comprises providing a motion signal to a CNC controller toinitiate the moving the torch. In some embodiments, the third set ofshield and plasma gases is distinct from the second set. In someembodiments, the inert gas of the second set increases a length of timerequired to fully pierce the workpiece. In some embodiments, the inertgas of the second set reduces a maximum plasma arc voltage required tofully pierce the workpiece. In some embodiments, the determining thatthe arc has fully pierced the workpiece comprises: estimating andmonitoring a characteristic of an electrical signal associated with thearc between the electrode and the workpiece; comparing thecharacteristic to a threshold value; and determining that an estimatedcharacteristic meets and/or exceeds the threshold value. In some cases,the characteristic comprises a rate of change of arc voltage or pulsewidth modulation duty cycle. In some embodiments, the characteristiccomprises a pulse width modulation control parameter.

In some aspects, plasma arc material processing systems can include aplasma arc torch including a torch body configured to receive a set ofconsumable components connected to the torch body and configured togenerate a plasma arc; and a power supply operably connected to thetorch, the power supply comprising: a means for calculating a pulsewidth modulation characteristic of an electrical signal associated withthe plasma arc between an electrode of a plasma arc torch and aworkpiece to be processed; a means for monitoring the characteristicduring operation of the torch over a time period of a workpiece piercingsequence; a means for comparing the characteristic to a threshold value;and a means for responsive to determining that a calculatedcharacteristic meets and/or exceeds the threshold value, ending theworkpiece piercing sequence and beginning the cutting sequence andcausing the plasma arc torch to move relative to the workpiece to form acut.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an example plasma arc torch.

FIG. 2 is a schematic representation of an example combined plasma gasand shield gas supply system to a plasma arc torch.

FIG. 3 is another schematic representation of an example combined plasmagas and shield gas supply system to a plasma arc torch.

FIG. 4 is a schematic circuit diagram of an example power control blockthat includes an arc voltage estimation module.

FIG. 5 is a schematic circuit diagram of another example circuit used toprovide electricity to form a plasma arc, depicting various measurementpoints to monitor torch performance.

FIG. 6 is a schematic depicting plasma arc voltage and a rate of changeof the voltage during an example cutting sequence, illustrating howvoltage reacts to physical changes affecting the cutting sequence.

FIG. 7 is a schematic of an example sequence for predicting an impendingplasma arc failure and implementing an arc extinguishing sequence priorto failure.

FIG. 8 is a schematic of an example sequence for predicting an impendingconsumable blow out in a plasma arc torch based on fluctuations inplasma arc current.

FIG. 9 is a schematic of example sequences for detecting plasma piercingworkpieces of different thicknesses by monitoring changes in electricalparameters of the system.

FIG. 10 is a schematic of example arc voltages during workpiecepiercing, illustrating reduced piercing voltages when using variousgases for the plasma and shield gases.

FIG. 11 is a schematic of example arc voltages during workpiecepiercing, illustrating reduced piercing voltages and shorter piercingtime for shorter standoff heights.

FIG. 12 is schematic diagram of another example combined plasma gas andshield gas supply system to a plasma arc torch, illustrating distancesbetween the torch and gas distribution valves.

FIG. 13 is a schematic depicting an example pulse width modulationcurrent control circuit, for example, as used in the HD 4070 PlasmaCutting System by Hypertherm.

FIG. 14 is a schematic depicting another example pulse width modulationcurrent control circuit, for example, as used in the HPR Plasma CuttingSystem by Hypertherm.

DETAILED DESCRIPTION

In some aspects, as discussed herein, the systems and methods describedherein can include a power supply having one or more precisely anddynamically monitored and controlled electrical and gas parameters ofthe plasma arc torch, including plasma arc current and pulse widthmodulation control thereof, plasma gas flow, and/or shield gas flow, canbe achieved using the system designs herein. The precise control ofthese systems has been found to be useful to implement severaladvantageous torch control methods described herein. Example methods, asdiscussed in detail herein, include ramp-down error detection andprevention sequences by detecting a lengthening of a plasma arc, moreaccurate consumable failure prediction by determining errors ordeviations in plasma arc current set points and actual plasma arccurrent and implemented torch protection sequences, and improvedworkpiece piercing sequences such as more accurate workpiece piercingdetection by calculating variations in electrical parameters (e.g.,changes in arc voltage or pulse width modulation values providingcurrent to the plasma arc).

Referring to FIG. 1, an example plasma arc torch 10 can include a torchbody 12 and a torch tip 11. The torch tip 11 includes multipleconsumables, for example, an electrode 14, a nozzle 16, a retaining cap13, a swirl ring 15, and a shield 22. The torch body 12, which has agenerally cylindrical shape, supports the electrode 14 and the nozzle16. The nozzle 16 is spaced from the electrode 14 and has a central exitorifice mounted within the torch body 12. The swirl ring 15 is mountedto the torch body 12 and can have a set of radially offset or canted gasdistribution holes that impart a tangential velocity component to theplasma gas flow, causing the plasma gas flow to swirl around theelectrode 14. The shield 22, which also includes an exit orifice, isconnected (e.g., threaded) to the retaining cap 13 or torch body. Thetorch 10 can additionally include electrical connections, passages forcooling, passages for arc control fluids (e.g., plasma gas), and a powersupply. In some examples, the torch can include a switch (e.g., atrigger switch) that can be used to send a signal to the power supply toinitiate generation of a plasma arc by the power supply. Based on thissignal, the electric components within the power supply may generate andmodulate the signal provided to the torch to control ignition andcutting operations.

In operation, plasma gas flows through a gas inlet tube (not shown) andthe gas distribution holes in the swirl ring 15. In some cases, thetorch can include a gas connection to fluidly couple the torch to ametering console. As discussed below, the metering console can be usedto provide gases, such as non-oxidative gases (e.g., inert gases (e.g.,argon gases)) to the torch for carrying out various processes. Fromthere, the plasma gas flows into a plasma chamber 18 and out of thetorch 10 through the exit orifices of the nozzle 16 and the shield 22.To operate the torch and generate a plasma jet, a pilot arc is typicallyfirst generated between the electrode 14 and the nozzle 16. The pilotarc ionizes the gas passing through the nozzle exit orifice and theshield exit orifice. This can be referred to as pilot arc mode. The arccan then be transferred from the nozzle 16 to a workpiece (not shown)for thermally processing (e.g., cutting, gouging, or welding) theworkpiece. It is noted that the illustrated details of the torch 10,including the arrangement of the components, the direction of gas andcooling fluid flows, and the electrical connections, can take a varietyof forms.

The torch 10 is typically electrically and fluidly connected to a plasmaarc system power supply to provide energy (e.g., electricity) to thetorch 10 to sustain a plasma arc during a desired torch operation. Apower supply can include a housing that serves as an enclosure forvarious electrical and fluidic components of the power supply. Forexample, various systems and devices can be implemented to deliver gasesand liquids to the torch in the form of a plasma gas delivered to theplasma chamber 18 between the electrode 14 and the nozzle in which aplasma arc can be formed or a shield gas between the nozzle 16 and theshield 22. For example, FIG. 2 is a schematic representation of acombined plasma gas and shield gas supply system to a plasma arc torch.A plasma gas programmable control valve 205, such as a proportionalsolenoid control valve, is positioned adjacent the torch 10 in theplasma gas supply line 201. A shield gas programmable control valve 405,such as a proportional solenoid control valve, can be positionedadjacent and/or in the torch 10 in the shield gas supply line 401.Optionally, a sensor 210 can be present in the plasma gas line 201 andprovide a control signal 212 to a DSP 215. Also optionally, a sensor 410can be present in the shield gas supply line 401 and provide a controlsignal 412 to DSP 215. The sensors 210, 410 can measure different typesof physical parameters, such as flow, pressure, and others, such asthose described above.

The precise and dynamic real-time control of the plasma arc torch,including plasma arc current and pulse width modulation control thereof,plasma gas flow, and/or shield gas flow, can be achieved using thesystem designs herein. The precise control of these systems has beenfound to be useful to implement several advantageous torch controlmethods described herein. Example methods described herein includeramp-down error detection and prevention sequences, more accurateconsumable failure prediction and detection and implemented torchprotection sequences, and improved workpiece piercing sequences such asmore accurate workpiece piercing detection or optimized.

Specifically, in a first aspect, the precise and dynamic control by thesystems described herein can be used to prolong electrode life byreducing plasma torch ramp-down errors. A ramp-down error (RDE) canoccur when the plasma system experiences a sudden loss of the plasmaarc, for example, when the plasma torch runs off of the workpiece and isunable to complete the coordinated ramp-down of plasma gas plenumpressure and cutting current (which in some cases is referred to as“Long Life Technology”). The sudden loss of the arc can cause increasedhafnium wear, especially in the presence of oxygen. For example, in somecases, without a proper arc extinguishing sequence, when the plasma arcis immediately lost, the high pressure of the plasma gas continuing toflow as if the plasma arc is still connected to the workpiece can blowaway molten emitter material, which can lead to the wear. Thus, preciseand accurate plasma gas ramping techniques can be especially useful forprolonging electrode life when the steady state cutting process istransitioned to torch shutdown (i.e., extinguishing the arc). That is,in this aspect, the power supply can predict/detect an undesired arcloss, which could otherwise cause unnecessary wear to the electrode, andtake action to limit such unnecessary wear to prolong life of theelectrode.

For example, when the plasma torch travels beyond the edge of thematerial, the plasma arc stretches (e.g., as the distance between thetorch and the edge of the workpiece (e.g., the arc attachment point)increases) and causes the PWM values of the system to vary accordingly.The increase in pulse width modulation duty cycle is a result of thechopper controller adjusting the pulse width modulator to maintain adesired current. Therefore, the systems herein can monitor theseelectrical, system, and control parameters to predict when a plasma arcis about to be lost. In response, the power supply can quickly takeaction to prevent sudden loss of the arc, which could result inelectrode wear and a shorter lifetime. As discussed below, this quickaction to prevent sudden loss of the arc can include adjustingelectrical parameters of the arc, gas flows to the torch, or motion ofthe torch itself. While a summary of an aspect of this application isprovided herein, more embodiments and details are provided below.

In a second aspect, as detailed below, the precise and dynamic controlby the systems described herein can be used to protect a plasma torchfrom damage that could result from failing consumables. That is, in someconventional devices, consumables, such as electrodes can wear to apoint of catastrophic failure. In some cases, a consumable can wearuntil it falls apart (e.g., an end of the electrode can blow off), whichcauses damage to the torch in which the electrode is installed. However,the inventors of the present application have discovered that as anelectrode approaches failure, variations in the plasma arc current(e.g., the set current vs. the actual current) can be observed. Thus bymonitoring electrical parameters of the plasma arc system, such asplasma arc current, the power supply can predict an impending consumablefailure and quickly take action to prevent such failure and thereby alsolimit damage to the torch. As discussed below, for example, the powersupply can monitor a current set point as compared to the actual currentof the plasma arc. Once an error between the set and actual currentreaches a threshold level, the power supply can take rapid action toprevent further damage. In some cases, for example, the power supply canshut down and lock from further use until the consumables are replaced.In some embodiments, as discussed below, fluctuations in coolanttemperature can additionally or alternatively be used to predict animpending consumable failure. That is, in some cases, rising coolantfluid temperatures can be indicative of impending consumable failure. Insome cases, arc current and coolant temperature can be used in acomplementary fashion in which the coolant temperature can be viewed asa warning indicator of a possible failure and the arc current canprovide a more conclusive notice of an impending failure. While asummary of an aspect of this application is provided herein, moreembodiments and details are provided below.

In a third aspect, the precise electrical and gas delivery systemsdescribed herein can be used to predict (e.g., using the power supplyitself) when a workpiece piercing sequence has completed so that thetorch can be moved and a material processing operation can begin. Byenabling pierce detection, various processing parameters can be adjustedto increase plasma torch performance. As detailed below, priorconventional systems typically pierce for a predetermined time beforeproceeding with moving the torch to perform a cut. This can result inunnecessarily long cutting times, unnecessary electrode wear, orundesired workpiece damage. However, in some embodiments, the arcvoltage estimation systems herein having precise and fast control can beused to detect when the plasma arc has successfully pierced theworkpiece to begin motion of the plasma torch and/or to alter the gasesdelivered for cutting. Accurately knowing once a workpiece has beenpierced can be useful to begin a cutting as soon as practical afterpiercing, instead of unnecessarily remaining in place after piercing, asis done in prior conventional systems. In some embodiments, as detailedbelow, the time when the arc pierces the workpiece can be detected bymonitoring one or more electrical signal(s) in a power supply generatinga plasma arc signal, such as a pulse width modulation parameter (or rateof change thereof) or a voltage parameter (e.g., rate of voltage changeover time (dV/dt)), one or more threshold values, or some combination ofboth.

In a fourth aspect, the precise and fast acting gas and electricalsystems described herein can be used to implement techniques duringpiercing to increase the usable life of an electrode (e.g., the numberof piercing sequences for which an electrode can be used). For example,performing a pierce sequence in the presence of a non-oxidative gas,such as an inert gas (e.g., an argon gas or a nitrogen gas), can help tocreate an environment in which the voltage needed to complete the pierceis reduced. As a result of the lower voltage for piercing, a plasma arcelectrode can be used for a greater number of piercing operations. Forexample, the inert gas (e.g., argon or nitrogen gas) and lower voltagescan permit the torch to be further away from the workpiece duringpiercing (e.g., have a higher standoff height), which can also helpincrease the usable life. For example, in some cases, the greaterstandoff height can help reduce molten workpiece material spatter fromadhering to the torch (e.g., consumables). As described herein, reducedgas volumes in the torch and gas valves that deliver shield and/orplasma gas to a torch can be located closer to the torch itself ratherthan at position far away (e.g., at the opposite end of a plasma leadline), as with some conventional systems. As a result of the valvesbeing close to the torch and the system working with smaller gasvolumes, the gases can be changed quickly and efficiently, making thegas delivery methods herein more feasible.

To achieve the inventive methods described in the aspects above, gasflow and electrical properties ramp up and ramp down techniques for theplasma and/or shield gases can be implemented, e.g., automatically,using DSP 215 (e.g., a CNC). In some embodiments, cut tables includingspecific cut information based on variables such as those identifiedabove can be used. These parameters can be stored, e.g., in electronicform, in a “cut table” for quick reference and utilization by the cutprogram. Thus, the cut table can store cutting shapes and parameters forready reference by the cut program or, e.g., for selection and use bythe equipment operator. Using such a system, automated control of torchcurrent levels and gas flow rates can be utilized for consistent cuttingoperations. Such a system can be employed to take advantage of specificcutting information that has been previously optimized in view of themany inter-related variables that affect cutting operations.

FIG. 3 is a schematic representation of another example combined plasmagas and shield gas supply system to a plasma arc torch, which providesenhanced operating flexibility. Specifically, FIG. 3 illustrates howmultiple (e.g., seven in this example) different supply gases can beincorporated into a system providing two cut gases 701, 702, each cutgas being available to provide plasma gas and/or shield gas. Forexample, different plasma or shield gas compositions can be preferredwhen cutting different workpiece materials or thicknesses. Additionallyor alternatively, in some embodiments, different plasma gas compositionscan be preferred for different aspects of a material processingsequence, such as a pilot arc initiation sequence (e.g., pilot arcmode), a workpiece piercing sequence (e.g., piercing mode), and asubsequent workpiece cutting sequence (e.g., cutting mode). In somecases, as detailed herein, one or more non-oxidative gases, such asgases comprising argon, can be used during a workpiece piercing mode,which can provide a beneficial environment for piercing a workpiece. Forexample, in some embodiments, a presence of argon gases during aworkpiece piercing sequence can be useful to reduce a plasma arc voltagerequired to fully pierce the workpiece. The reduced voltage required canalso be useful for preserving or extending the usable life of the plasmaarc electrode by increasing the number of piercing sequences that anelectrode can undergo before failure.

In some embodiments, as depicted in the example system FIG. 3, a systemcan supply a gas having a first, high argon content (e.g., H5 (about 5%hydrogen content, about 95% argon)) via a valve (e.g., a solenoid valve)710, a gas having a second, lesser argon content (e.g., H35 (about 35%hydrogen content, about 65% argon content)) via a valve 711, a gashaving a high nitrogen content (e.g., F5 (about 5% hydrogen content,about 95% nitrogen)) via a valve 712, and/or methane (CH4) via a valve713 as a first cut gas (e.g., as a plasma gas) 701. Nitrogen (N2), forexample, as the first cut gas, can also be supplied via a valve 714. Aircan also be supplied as the first cut gas via a valve 715. A check valve703 can be included in the first cut gas line. This first cut gas 701can be channeled through a plasma gas programmable control valve 205positioned adjacent the plasma torch, and through sensor 210 asdescribed above. While certain example gases have been described abovefor the first cut gas (e.g., the plasma gas), these are provided asexamples only. Thus, the systems and methods described herein can beused to deliver any of various gas combinations to the plasma torch foruse. Additionally, the various valves can be used to provide differentgases to the torch during different cutting sequences.

Nitrogen can also be supplied as a second cut gas (e.g., a shield gas)702 via on-off solenoid valve 720, air via a valve 721, helium via avalve 723, and 02 (oxygen) via a valve 722. A check valve 704 can beincluded in the second cut gas line. As illustrated, the second cut gascan be used as a shield gas, which can pass through a shield gasprogrammable control valve 405 positioned adjacent to the plasma arctorch, and through sensor 410 as described above. Although not shown, aDSP 215 can be used to manipulate, e.g., the programmable control valves205, 205A, 405, and 405A, and/or some or all of the valves 710, 711,712, 713, 714, 715, 720, 721, 722, and 729. The valves described hereincan include various types of valves, such as selection valves orthree-way valves. Additionally, the valves described herein can haveimproved response times, such as on the order of about 5 milliseconds toabout 40 milliseconds. It is noted that the improved valve responsetimes, in addition to reduced gas volumes within the torch, can improveconsistency of the torch operation, such as switching gases delivered tothe torch or implementing torch shut-down sequences.

While certain example gases have been described above for the second cutgas (e.g., the shield gas), these are provided as examples only. Thus,the systems and methods described herein can be used to deliver any ofvarious gas combinations to the plasma torch for use. Additionally, thevarious valves can be used to provide different gases to the torchduring different cutting sequences. Moreover, while certain gases havebeen described as being possible for the first cut gas or the second cutgas, one skilled of the art would understand that the various examplegases, or other gases described herein, can be used as first cut gasesor second cut gases according to the various sequences described hereinand in the claims.

Some examples, as depicted in FIG. 3, can also feature crossover lines730 and 732. That is, the crossover line 730 delivers the first cut gas(e.g., the plasma gas) 701 to be used as a shield gas; and the crossoverline 732 delivers the second cut gas (e.g., the shield gas) 702 to beused as a plasma gas. Each of the crossover lines can include valve(e.g., a programmable control valve) 205A, 405A, such as a proportionalsolenoid control valve, which can be used to control the amount ofcrossover gas flow. For embodiments incorporating crossover flow, it istypically useful that crossover programmable control valves 205A, 405Abe positioned adjacent or within the plasma arc torch. Crossover gasflow can be used to augment or replace other gases already flowing inthe system.

Although the above discussion has focused primarily on programmablecontrol valves (e.g., 205, 405) that are located adjacent the torch(e.g., within 2 feet, 6 feet, or 10 feet of the torch), some embodimentsalso include torches into which the programmable control valvefunctionality has been integrated. For example, the plasma torch and aprogrammable control valve can be a unitary assembly, i.e., with noconnecting hose between the two. Such closely-coupled embodimentsinclude a programmable control valve that is directly attached, i.e.,directly coupled (e.g., bolted) to the housing of a plasma torch. Insome embodiments, the programmable control valve apparatus is disposedwithin the plasma torch body. Locating programmable control valvefunctionality (e.g., for the plasma gas or the shield gas) within thetorch reduces the number of external components present in a plasmatorch system.

While some example gas delivery system configurations have beendescribed above, other embodiments are possible. Specifically, whilesome examples are described in which gases used for operating the plasmaarc torch (e.g., plasma or shield gases) are located off of (e.g.,substantially away from (e.g., at an opposite end of a torch lead hose))the torch, other examples can include gas supplies located at differentlocations. For example, in some embodiments, gases used to assist inworkpiece piercing (e.g., argon) can be located closer to the torch thansome other gases. In some cases, locating a pierce assist gas closer tothe torch than at the opposite end of the torch lead hose relative tothe torch, such as at metering console fluidly connected to the torch,can be used to better control gas delivered to the torch during apiercing operation. For example, the metering console can controldelivery of the pierce assist more quickly and more precisely than someconventional gas delivery techniques. The metering console isillustrated and described in more detail below with respect to FIG. 12.

Also illustrated in FIG. 3 is an optional vent valve 729 that can beused to vent plasma gas to the atmosphere (ATM). A vent valve 729 can bean on-off solenoid valve. In some embodiments, a vent valve 729 can be aprogrammable control valve. A programmable control valve vent valve 729can be useful when cutting conditions are encountered in which it isdesirable to decrease the flow rate of plasma gas flowing to the plasmachamber 207 more rapidly than is possible with only the plasma gasprogrammable control valve(s) 205, 205A (e.g., to improve ramp-downtimes). In some embodiments, vent valve 729 can be two or more on-offsolenoid valves mounted in parallel, providing for increased ventingcapacity and control. The two or more vent valves can be operatedindependently or simultaneously and can have different sizes (i.e.,different valve coefficients (C_(v)'s)).

Of course, other combinations are possible. Gas supplies other thanthose illustrated can be used, and various techniques known to those ofskill in the art can be used, e.g., to mix different gas sources. Forexample, a plasma gas mixture can be formed by mixing methane and H35using techniques and instrumentation (not shown) known to the skilledartisan. In addition to the mixing techniques described above usingcrossover lines 730 and 732, additional gas supplies can be addedadjacent the torch and these additional gas supplies can be controlledusing programmable control valves. For example, some embodiments includeadding a reducing gas stream (such as methane) to the shield gas betweenprogrammable control valve 405 and sensor 410, such that the flow of thereducing gas is controlled with a programmable control valve. More thanone additional gas (such as the reducing gas stream) can be added inthis manner. Similar techniques can be used for the plasma gas. Manyother arrangements and combinations are also possible.

Another example gas delivery configuration is depicted in FIG. 12. Asillustrated, the system can provide various gases, such as F5, H2, Ar,N2, O2, and air to several valves fluidly connected to the torch toprovide shield and plasma gases. As depicted, the valves (e.g.,three-way valves) that direct gas to the torch can be disposed in atorch connect module that is about 2 meters to about 4.5 meters from thetorch. Whereas, the gas supplies can be disposed about 3 meters to about15 meters from the torch connect module.

As discussed below, gases can be delivered to the torch, for example asplasma gases or shield gases, in various sequences in order to implementthe torch operation techniques described herein, including torchinitiation, workpiece piercing, or cutting sequences. Unless otherwisestated herein, the various gas delivery methods and sequences below canbe implemented using the above described gas delivery systems.

Alone or in combination with the various gas delivery systems describedherein, electrical systems for controlling and operating a plasma arctorch can be used to estimate (e.g., or determine) one or moreelectrical characteristics, which can be used to implement one or moreprocessing sequences. In some embodiments, for example, the plasma arcsystem can estimate certain physical characteristics of a cuttingsequence and implement a response to such physical characteristics. Forexample, FIG. 4 is a circuit diagram of a power control block of a powersupply that includes an arc voltage estimation module. As shown, thecircuit 100 includes a Pulse Width Modulation (PWM) control circuitblock 200 coupled to a power circuit block 300. The power circuit block300 can be a switched mode power supply that includes an unregulatedinput voltage source V_(IN), a power transistor switch-diode combinationQ1, D1, an output filter inductor L1 and a plasma arc load R_(LD). Thepower circuit block 300 can operate as a standard chopper such that theoutput current I_(ARC) through the arc load R_(LD) depends on the dutycycle of the switch Q1. Although the power circuit block 300 shown is abuck converter, other embodiments can include other circuit topologies,including boost, buck-boost and variations thereof.

The PWM control circuit block 200 can provide a gate signal T3PWM to theswitch Q1 to control its duty cycle, and thus the output current I_(ARC)through the plasma arc load R_(LD). As shown, the PWM control block 200includes a current reference block 210, an error control block 220, afeedback current sensor 240, a PWM comparator block 230, and an arcvoltage estimation module 250.

A program or an operator of the system can manually set block 210 to adesired current reference I_(REF) at which to maintain the outputcurrent I_(ARC). The output current IARC may be monitored using thecurrent sensor 240, such as a Hall current sensor. The current sensor240 transmits a feedback current I_(FB) to an input of the error controlblock 220. The error control block 220 can be implemented, for example,as a standard proportional-integral-derivative controller (PIDcontroller) known to those skilled in the art. The error control block220 compares the feedback current I_(FB) against the desired currentreference I_(REF) and outputs a modulating error signal, Error.

The error signal, Error, can then be input to the PWM comparator block230 where it is sampled and used to generate the appropriate gate signalT3PWM that adjusts the duty cycle of the switch mode power supply 300,thereby correcting for the error in the output current. The PWMcomparator block 230 and the arc voltage estimation module 250 can berealized using a digital signal processor (DSP), such as TMS320LF2407 Afrom Texas Instruments. These control blocks can also be realized usinga combination of one or more suitably programmed or dedicated processors(e.g., a microprocessor or microcontroller), hardwired logic,Application Specific Integrated Circuit (ASIC), or a Programmable LogicDevice (PLD) (e.g., Field Programmable Gate Array (FPGA)) and the like.

In order to generate the appropriate gate signal T3PWM, the PWMcomparator block 230 compares an instantaneous error sample T3CMPR witha carrier wave signal T3CNT. The carrier wave signal can be generated asa sawtooth or triangular carrier wave with its frequency ranginganywhere from hundreds of Hertz (Hz) to MegaHertz (MHz) depending on theapplication. In a plasma cutting application, the frequency of thecarrier wave signal is typically around 15 kHz. The comparator amplifiesthe difference between the two signals and produces a gate signal T3PWMwhose average value over one switching cycle of the carrier wave signalT3CNT is equal to the value of the instantaneous error sample T3CMPR.Application of the gate signal to the switch Q1 adjusts the duty cycleto drive and maintain the output current I_(ARC) at a desired steadystate value.

Additionally or alternatively to the arc estimation techniques describedabove, other techniques are possible for estimating the electricalcharacteristic(s) of the plasma arc. For example, in some embodiments,the duty cycle of a pulse width modulator that provides an electricalsignal to the plasma arc torch to create the plasma arc can be monitoredand used to estimate any of various physical implications observedduring use of the plasma arc torch.

For example, referring to FIG. 5, in some cases, a power supply system500 can include a transformer 502 that receives an electrical input(e.g., a line voltage) and converts it into a system signal, such as abus voltage (e.g., a direct current bus voltage). In some cases, the busvoltage can be at or about 360 volts.

From the transformer 502, the bus voltage can feed to a chopper 504. Thechopper 504 can include a switching device that converts fixed DC input(e.g., the bus voltage) to a variable DC output voltage used to generatethe electrical signal for the plasma arc. The variable DC output voltagefrom the chopper 504 can be provided to the torch and used to generatethe plasma arc to perform a plasma arc processing operation 506, forexample, a cutting operation through a workpiece.

Electricity provided to the workpiece for the processing operation(e.g., the electricity that forms the plasma arc) is transferred throughthe workpiece and returned back to the power supply system. For example,a closed circuit can be formed with the chopper to maintain theelectrical circuit with the workpiece and the power supply system. Forexample, a pulse width modulation system 508 can be disposed in circuitbetween the workpiece and a return to the chopper 504. The pulse widthmodulation control system 508 can be implemented to set a current level(e.g., by adjusting its duty cycle) for the electrical signal thatgenerates the plasma arc. A pulse width modulation (PWM) signal is amethod for generating a signal (e.g., an analog signal) using a digitalsource. For example, the chopper can have an open circuit voltage andduring operation the current can be measured and if the current is toolow, the pulse width modulation duty cycle can be increased. Thissampling and adjustment can be performed periodically (e.g., every 33microseconds) to adjust the pulse width modulation duty cycle. Thisadjustment can be viewed as a control loop, and the adjusted pulse widthmodulation duty cycle can be compared at each loop to the thresholdvalue. For example, a PWM signal consists of two main components thatdefine its behavior: a duty cycle and a frequency. The duty cycledescribes the amount of time the signal is in a high (on) state as apercentage of the total time it takes to complete one cycle. Thefrequency determines how fast the PWM completes a cycle (i.e., 1000 Hzwould be 1000 cycles per second), and therefore how fast it switchesbetween high and low states. By cycling a digital signal off and on at afast enough rate, and with a certain duty cycle, the output will appearto behave like a constant voltage analog signal when providing power todevices. As one skilled in the art would understand, the pulse widthmodulation control system duty cycle is akin to a throttling mechanismto maintain a set current. That is, in some examples, pulse widthmodulation is a percent time “on” for the system. For example, systemscan be operates at 15 kHz which is a time period of 66 microseconds, thePWM is the percent on time. For example, to create a 3V signal given adigital source that can be either high (on) at 5V or low (off) at 0V,one can use PWM with a duty cycle of 60% which outputs 5V 60% of thetime. Thus, there the signal is “on” for 60% of the time (e.g., a 60%“on” time). If the digital signal is cycled fast enough, then thevoltage seen at the output appears to be the average voltage. If thedigital low is 0V (which is usually the case) then the average voltagecan be calculated by taking the digital high voltage multiplied by theduty cycle, or 5V×0.6=3V. Selecting a duty cycle of 80% would yield 4V,20% would yield 1V, and so on.

In addition to controlling a current set point for the plasma arc, whichsets the intended current of the plasma arc, the actual current of theplasma arc can be monitored. For example, the current at an electricalconnection between the plasma arc and the pulse width modulation controlsystem 508. This electrical connection is marked in FIG. 5 as currentmeasurement point 510. As one skilled in the art would readilyunderstand, voltage can also be measured where the current is measured.

As discussed below, electrical characteristics can be measured andmonitored at various points throughout the system to predict orotherwise estimate changes (e.g., physical or mechanical changes) in aplasma arc process being implemented. For example, fluctuations inplasma arc voltage or in the pulse width modulator duty cycle supplyingthe plasma arc can indicate a lengthening of the plasma arc.Additionally, plasma arc current can be monitored (e.g., at the currentmeasurement point 510) and compared to a current point, by whichconsumable failure can be predicted by noise levels between the setpoint and the actual current.

Additionally, in some embodiments, pulse width modulation (PWM) currentcontrol in a power supply can implemented using a hybrid combination ofDSP and op-amp based circuits. For example, first, the building blocksthat include a PWM Current Control circuit (e.g., the HD 4070 PlasmaCutting System from Hypertherm) are described in FIG. 13. Thisdescription is then followed with a description of the HPR PWM CurrentControl circuit in FIG. 14. The use of a DSP in the HPR control circuitfacilitates easy estimation of the arc voltage. In addition to strictlyusing arc voltage, as described herein, the PWM duty cycle can also becalculated and used to implement the various methods herein. Forexample, FIG. 13 shows the schematic of an example plasma cutting powersupply control system, which includes an analog op-amp based PWM currentcontrol circuit and its associated power circuit block. The buildingblocks of the PWM control circuit can include:

-   1) An error controller block (e.g., Proportional-Integral-Derivative    or PID) that monitors the power supply current and produces an error    output after comparison with a desired current reference;-   2) A PWM comparator block that compares a modulating signal (in this    case the error controller output) with a carrier wave signal. The    latter is generally a saw-tooth or triangular waveform with its    frequency ranging anywhere between 100's of Hz to MHz depending on    the application. In a plasma cutting application, this frequency is    typically around 15 kHz. The comparator amplifies the difference    between the two signals and produces an output whose average value    over one switching (carrier wave) cycle is equal to the value of the    modulating wave at the instant of comparison; and-   3) A feedback current sensor block (e.g., Hall current sensor) that    monitors the power supply output current and feeds it back to the    controller.

The plasma cutting power circuit block can include an unregulated dcinput voltage source V_(in), a power transistor switch-diode combination(Q₁-D₁), an output filter inductor (L₁) and the arc load. The PWMcomparator output controls Q₁ state (ON or OFF) to fulfill the controlobjective of achieving and maintaining the desired output current.

In another example, as depicted in FIG. 14, which is a block diagram ofa Hybrid Plasma Cutting Power Supply Control System (e.g., HPR 130 PowerSupply by Hypertherm). In this example, the PWM comparator block can berealized using a DSP (TI LF 2407). The appropriate peripherals insidethe DSP chip are typically configured to produce a triangular carrierwave of 15 kHz frequency. An op-amp based PID error controller outputcan be sensed through the DSP A/D converter interface. The PWM signalcomparator then compares these signals and generates a PWM pulse trainat its output. This signal is then used to turn-on and off the HPRchopper IGBT controlling the output current in the process.

Using these and other power supply designs, the arc voltage can becalculated in several ways. For example, in some embodiments, methods toestimate arc voltage can be based on the principle that inductor voltagedrop is substantially zero at constant arc current. This implies thatthe average dc voltage at the input of the inductor is equal to theaverage value of the arc voltage. For example:V _(arc) =D _(ss) *V _(in)  Eq. (1)

Where,

D_(ss)-steady state duty cycle of the transistor switch

V_(in)-unregulated dc input voltage

V_(arc)-average arc voltage component

In the case of the HPR control circuit both of these parameters (i.e.D_(ss) and V_(in)) are readily available for arc voltage computation.The current controller output whose value is stored by the DSP in theT3CMPR (Timer compare register) is the duty cycle value D_(ss). The DSPcan also monitor the input ac voltage V_(acin) to provide input ac underand over-voltage protection to the system. This parameter can be used toobtain V_(in) since the two (V_(in) and V_(acin)) are directlyproportional to each other. With both of these parameters available, Eq.(1) can be easily implemented in the DSP with very little computationaloverhead for V_(arc) estimation.

This type of arc voltage estimation can have several advantages. First,for example, the arc voltage estimation implementation does not requireadditional hardware and requires very little software and computationaloverhead. Also, the estimated V_(arc) value can be transmitted to aTorch Height Controller, which can set the standoff height of the torchfrom the workpiece, through a serial cable already linking the two(i.e., power supply with the THC). In the process, the voltage dividerboard currently used for arc voltage measurement as well as cablesconnecting this board to the power supply and the Torch Command Module(TCM) can be eliminated reducing associated board and wiring costs.Additionally, arc voltage measurements using the voltage divider boardcan be susceptible to electrical noise even though the divider boardprovides a galvanically isolated measure of the arc voltage. Whereas,the arc voltage estimation herein reduces (e.g., eliminates) thisproblem and its associated servicing costs.

The above electrical and fluid management systems can be used toimplement various methods to implement or improve a material processingoperation, such as a cutting or gouging operation.

Consumable Protection Methods

Developments in the electrical systems described herein, such as plasmaarc power supply systems, have increased the ability to quickly monitorvarious electrical signals indicative of a physical state of the plasmaarc (and thus also of an impending arc failure event) and also theability to respond quickly to generate an electrical arc ramp procedure.That is, electrical characteristics observed during a materialprocessing operation, such as a cutting, gouging, or other materialaltering operation, can be used to infer various issues encounteredduring the operation and, based on the characteristics, the plasma arcsystem can implement various responsive actions to maintain theprocessing operation or to limit undesired damage to system components,such as consumables of the plasma torch.

For example, the systems and methods described herein can be used toreduce plasma torch ramp-down errors. Plasma cutting electrode life canbe significantly reduced when subjected to ramp-down errors. A ramp-downerror (RDE) can occur when the plasma system experiences a sudden lossof the plasma arc and is unable to complete the coordinated ramp-down ofplasma gas plenum pressure and cutting current (which in some cases isreferred to as “Long Life Technology”). The sudden loss of the arc cancause increased hafnium wear, especially in the presence of oxygen. Forexample, in some cases, without a proper arc extinguishing sequence,when plasma arc is immediately lost, the high pressure of the plasma gascontinuing to flow can blow away molten emitter material, which can leadto the wear. Also, in some cases, the arc snapping out can generateturbulent flows within the torch, also resulting in material loss. Insome cases, a ramp-down error RDE rate of 20% can result in electrodelife reductions of 50% or more. Thus, the systems and methods describedherein can be used to reduce plasma torch ramp-down errors.

The concepts described herein can enable a plasma system to detect theearly stages of ramp-down error, for example, by monitoring anelectrical characteristic, such as the estimated arc voltage, changes inestimated arc voltage, a characteristic of the pulse width modulationcontrol system (e.g., duty cycle or rate of change of duty cycle), andthen initiate a rapid ramp-down. For example, in some cases, the rapidramp-down can include a rapid current and/or gas ramp down.Alternatively or additionally, the ramp-down can include stopping amotion of the torch using the torch controller (e.g., a CNC). Thedetection and subsequent ramp-down will typically occur before the powersupply reaches its maximum voltage capability (e.g., the point at whichthe plasma arc is lost), limiting or preventing ramp-down error byadjusting a number of system parameters to tailor a ramp down profilefor the specific event.

Arc Voltage Behavior Before Ramp-down Error

When the plasma torch travels beyond the edge of the material, theplasma arc stretches (e.g., as the distance between the torch and theedge of the workpiece increases (e.g., the arc attachment point wherethe plasma arc attaches to the workpiece)) and causes the arc voltage toincrease rapidly. Along with the arc voltage, the pulse width modulationduty cycle would also increase as the chopper controller adjusts thepulse width modulator to maintain a desired current. Typically, thiswould occur at either the external edge of the material or on the edgeof a large internal feature ((e.g., a kerf, a hole, a gap, etc.) (largeenough such that the arc cannot “jump” the gap)). While the pulse widthmodulation duty cycle and arc voltage are expected to vary slightlyduring steady-state cutting, a stretched arc will cause them to abruptlyand significantly increase.

Detecting An Impending Ramp-down Error

The arc voltage may be monitored directly or calculated using the BUSSvoltage and PWM duty cycle of the current control system. Lower actionlimits can be used to filter normal voltage fluctuations due to materialimperfections, a warped plate, arc dynamics, etc. Action limits may alsobe imposed to filter out kerf-crossing voltage changes. In FIG. 6, therelationship between V_(kerf) and V_(edge), V, and dV/dt may allpotentially indicate unique characteristics at an edge and may be usedto differentiate between a recoverable kerf crossing and anunrecoverable edge. Additionally or alternatively, as mentioned above,the pulse width modulation duty cycle maintaining a desired currentlevel for the plasma arc can also indicate the unique characteristics atthe edge. The example of FIG. 6 depicts the behavior of a 130Aoxygen/air process at 20 inches per minute (ipm). This is a very slowcutting speed and the voltage trace is similar to the predictedbehavior. This test also shows the pierce event (i.e., where the voltageand rate of change of voltage spikes), which would typically need to beaccounted for by adjusting the arc voltage action limit, for example, tolimit or avoid false ramp-down estimation. While similar behavior isexpected for other cutting scenarios, as the cutting speed increases,dV/dt also typically increases, resulting in less time for ramp-down. Asa result, filtering kerf crossings (i.e., to avoid false ramp-downs ifit is not desired by the operator to ramp down when crossing a kerf) maybe more challenging as speed increases.

Therefore, referring to FIG. 7, electrical characteristics can bemonitored to predict when a plasma arc is about to be inadvertentlyextinguished (e.g., in the absence of a planned arc extinguishingsequence) and implement a substitute (e.g., accelerated) arc shut down.The characteristic and changes thereof can suggest physical changes thathappen during the processing operation. Specifically, fluctuations inthe electrical characteristics of the plasma arc can suggest physicalchanges in the plasma arc, such as a lengthening of the arc. In somecases, such lengthening of the arc can suggest that the plasma torch hasgone, or is about to go, over a void in the workpiece. For example, thevoid can include running off an edge (e.g., an outer peripheral edge) ofa workpiece or running over a kerf. For example, in some aspects,methods, such as methods for limiting plasma arc blowout (e.g., or snapout) or for preserving a usable life of a plasma arc electrodeconsumable installed in a plasma arc torch can first include measuring(e.g., continuously, instantaneously, or in real time measuring) acharacteristic of an electrical signal being provided to the torch togenerate a plasma arc between the torch and a workpiece to be processed.That is, as the plasma torch begins to leave the workpiece and movetowards the void, the measured characteristic, or a change thereof, canindicate an increasing distance between the plasma arc torch and the arcattachment point where the plasma arc attaches to the workpiece. Forexample, the characteristic typically includes a characteristic of apulse width modulation system (e.g., the pulse width modulation controlsystem 508). In some cases, the characteristic can include a duty cycleof the pulse width modulation control system. That is, the pulse widthmodulation duty cycle would also increase as the chopper controlleradjusts the pulse width modulator to maintain a desired current as thearc gets longer. Alternatively or additionally, the characteristic caninclude a rate of change of the duty cycle of the pulse width modulationcontrol system. The duty cycle of the pulse width modulation controlsystem can also suggest or indicate an amount of electrical force beingused to maintain a consistent (e.g., constant) plasma arc current. Ineffect, the duty cycle can show the level of effort needed to control aconsistent plasma arc current. That is, using the electrical systemdesigns described herein, this monitoring and control of the system andphysical state of the arc can be performed in the power supply itself,which allows for faster detection and response accordingly and does notneed to rely on the controller for this information). Similarly,performance with the workpiece can be monitored in near real time andvia the communication systems and the improved gas control system,alternative ramp-down approaches can be implemented to lengthen life. Inother prior conventional systems, the controller has typically been usedto monitor torch performance and make adjustments accordingly. As aresult, reaction times using the systems and methods herein can befaster, and thus more protective of torch components, than priorconventional systems.

In some embodiments, the characteristic can include a voltage of theplasma arc. For example, the plasma arc voltage can be estimated (ordetected) in accordance with the systems and methods illustrated anddescribed above with respect to FIGS. 4 and 5.

These methods typically also include monitoring the characteristicduring operation of the torch over a time period and comparing thecharacteristic to a threshold value. For example the time period overwhich the characteristic is monitored can include a material processingtime period (e.g., time to complete a cutting or gouge operation) or apredetermined time period. As mentioned above, the characteristic cancorrelate to a physical observation of the plasma arc and thresholdvalues can be used to estimate certain physical criteria of the plasmaarc. For example, certain values of pulse width modulation duty cycles,plasma arc voltages, or rates of change thereof can suggest that aplasma arc has reached a certain (e.g., undesirable) length. Thus, itcan be useful to determine such threshold values (e.g., empirically) tobe able to predict characteristics of the arc itself.

In some examples, the threshold value can be predefined for a particularcutting process or material. That is, the threshold can vary betweendifferent types of workpiece materials and/or thicknesses, differentcutting speeds, or different cutting operations (e.g., fine cutting,rough cutting, gouging, etc.). In some cases, the plasma arc system(e.g., the power supply) can reference a lookup table based on theprogrammed cutting operation to determine the desired threshold to beconsidered. For example, threshold pulse width modulation duty cyclevalues can be greater than about 70% (e.g., greater than or equal toabout 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%), greater than about80% (e.g., greater than or equal to about 85%), greater than about 90%,or greater than about 95%.

Based on the comparison of the characteristic to the threshold, theplasma arc system can implement various adjustments to the materialprocessing operation. For example, in some embodiments, in response todetermining that a measured characteristic meets (e.g., and/or exceeds)the threshold value, the plasma arc system can initiate an arcextinguishing sequence. In some cases, the arc extinguishing sequence(e.g., also referred to as a plasma arc shut down sequence) can preserveor protect the life of the electrode prior to a failure of theelectrode. That is, the system can, based on the measuredcharacteristic, predict when an undesired or inadvertent arc failure orother system shut down is about to occur and implement an accelerated(e.g., emergency or rapid) shut down sequence. For example, the gaspressure and current can be decreased rapidly to a level that willreduce (e.g., minimize) hafnium erosion of the electrode emitter. Insome embodiments, this can result in improved electrode life by reducingthe amount of hafnium ejected (blown off) as a result of arc snap outs,blow outs, or extinguishment. Similar to the standard ramp-down process,low plasma plenum gas pressure is typically desired to reduce (e.g.,minimize) hafnium erosion. This can be particularly useful becauseinadvertent arc shut down can cause damage to the torch consumables(e.g., degradation of the electrode), so if the system predicts that thearc is about to be shut down, for example because the torch is going togo off the edge of the workpiece, it can implement a shortened oraccelerated shut down procedure. In some embodiments, the arcextinguishing sequence can include decreasing a plenum gas pressure sothat the plasma arc is extinguished in a lower pressure environment. Forexample, in some cases, the plenum gas pressure can be decreased toabout 7 psi to about 3 psi. The pressure can be decreased quickly in anyof various ways. For example, concepts to reduce the time to reduceplasma gas pressure can include using additional exhaust paths (vents)near the torch or reducing one or more supply gas lines to atmosphericpressure while cutting and allowing the plasma plenum to exhaust intothe lower-pressure supply gas line.

As previously mentioned, conventional electrode life can be reduced by50-70% when subjected to a ramp-down error rate of 20%. Someconventional designs and some new concepts exhibit this vulnerability.If ramp-down error is minimized or prevented, the electrode life will bemuch more consistent and predictable

Additionally or alternatively, in some embodiments the arc extinguishingsequence can include decreasing a plasma arc current so that the plasmaarc is extinguished in a lower current environment. In some embodiments,the plasma arc current can be decreased to a value of about 90 amps toabout 22 amps. In some examples (e.g., high current examples), thecurrent is decreased from about 300 amps down to about 90 amps. In someexamples (e.g., low current examples), the current can be increased fromabout 30 amps to about 22 amps. In some examples, the current candecrease by about 25% to by about 70%. In some cases, the plasma arccurrent can be decreased to near zero well within the expected 100 mstimeframe.

As described above, the arc extinguishing sequence can be implemented toquickly extinguish the plasma arc but in a controlled manner. The amountof time over which the shut down sequence can take place can vary. Forexample, the lower action limit, signal delay, torch travel speed,maximum sustainable arc voltage, and gas valve delay typically determinethe amount of time available for ramp-down. In some embodiments, the arcextinguishing sequence can be completed in less than about 70milliseconds (e.g., less than about 60 milliseconds (e.g., less thanabout 10 milliseconds)). For example, in a 300 amp cutting process, thearc extinguishing sequence can be completed in less than about 60milliseconds. In another example, in a 30 amp cutting process, the arcextinguishing sequence can be completed in less than about 10milliseconds. In one example, for a predicted maximum voltage of about300V, a cutting speed of 20 ipm to 230 ipm, and a CAN signal delay of 10ms, the predicted time available for ramp-down can range from about 100ms to about 1000 ms.

Other methods for operating a plasma arc torch can implement one or moreof the above concepts for extinguishing a plasma arc during a materialprocessing (e.g., cutting or gouging) operation to limit or reduceelectrode wear resulting from unintended arc loss.

Such methods can include initiating the plasma arc between an electrodeof the plasma arc torch and a workpiece to be processed with the torch(or transferring a plasma arc from a nozzle to the workpiece) andtranslating the torch along the workpiece and performing a processingoperation to the workpiece. For example, the plasma arc can be initiatedto begin piercing or cutting/gouging and the torch can be moved to formthe cut or gouge in the workpiece.

The torch can be advanced or otherwise moved along the workpiece andtowards a void defined by the workpiece. As mentioned above, this caninclude moving the torch during a material processing operation towardsan edge of the workpiece (e.g., an outer peripheral edge) or a kerfformed by a prior processing operation. In some cases, the torch may bemoved towards the void (e.g., an edge or kerf) intentionally due to thedesired processing operation. Alternatively, in some cases, the torchmay be moved towards the void inadvertently, for example, because theworkpiece is not sized properly or is mispositioned on a plasma arcsystem (e.g., a cutting table). In some cases, the plasma arc system maybe directing the torch along the workpiece and undesirably reach anunexpected edge of the workpiece.

As discussed above, as the torch reaches or goes off of the void, theplasma arc can increase in length between the torch and the arcattachment point at which the arc connects from the electrode to theworkpiece. Thus, responsive to the torch reaching the void, the methodcan include detecting a lengthening of the plasma arc between the arcattachment point and the torch. In some embodiments, the detecting thelengthening of the plasma arc comprises monitoring a characteristic ofan electrical signal being provided to the torch to generate the plasmaarc and comparing the characteristic to a threshold value. As mentionedabove, the characteristic can include any of various electrical criteriaassociated with the plasma arc. For example, the characteristic caninclude a bus voltage of the plasma arc system. In some cases, the busvoltage can be compared to an incoming voltage to determine a duty cyclepercentage of at least one of a chopper or a pulse width modulator.These plasma arc estimation and pulse width modulation duty cyclecalculations are described above in detail. The characteristic caninclude a pulse width modulation value, such as duty cycle. For example,in some examples, the chopper output can be determined. The chopper canhave an open circuit voltage and during operation the current can bemeasured and if the current is too low, the pulse width modulation dutycycle can be increased. This sampling and adjustment can be performedperiodically (e.g., every 33 microseconds) to adjust the pulse widthmodulation duty cycle. This adjustment can be viewed as a control loop,and the adjusted pulse width modulation duty cycle can be compared ateach loop to the threshold value. The characteristic can also include arate of change of an electrical measurement, such as a rate of change ofthe pulse width modulation duty cycle or a rate of change of a plasmaarc voltage. In some cases, using the rate of change can be useful as away of filtering, removing false positives, or otherwise looking passedtemporary spikes or fluctuations that may not necessarily indicate suchthe lengthening of the plasma arc. Based on the detected plasma arclengthening, the plasma arc system can initiate an arc extinguishingsequence, which can provide a more desirable environment to extinguishthe plasma arc and limit wear of the electrode.

Unless otherwise stated, various features and embodiments of thesemethods can be combined with one another or with the other aspectsdescribed herein.

Torch Protection Methods

Electrical measurements relating to the plasma arc or other observedelectrical signals within a plasma arc power supply relating to theplasma arc can also be used as a way to monitor for consumabledegradation or failure and protect a torch body. That is, in some cases,as a consumable installed within a torch body begins to fail andcompletely blows out (e.g., damage can occur through arcing damage or inmolten copper/materials which gets into the cooling passages of thetorch), the torch body can also be damaged due to the high heat andfluid loss from the consumables. However, referring to FIG. 8, utilizingsome of the torch monitoring and control systems described above, torchoperation can be monitored in order to detect or predict an impendingtorch failure and end the torch operation to limit or prevent damage tothe torch.

For example, in some aspects, methods for limiting damage to a plasmaarc torch body resulting from a detected of an impending consumablefailure within the torch can include determining a specifiedconductivity parameter (e.g., current or voltage) set point of a currentto be provided to the plasma arc torch for a material processingoperation. For example, with regards to the embodiments described aboveand illustrated in FIGS. 4 and 5, a pulse width modulation controlsystem 508 can be used to set a desired plasma arc current set point.The current set point can vary based on various limitations, such as thedesired cutting process (e.g., fine cutting, gouging, etc.), desiredcutting speed, material to be processed, etc.

Next, a conductivity parameter of plasma arc current being provided tothe plasma torch to perform the material processing operation can bemeasured. That is, while setting the desired plasma arc current orvoltage, the system can also measure or otherwise determine the actualcurrent or voltage of the plasma arc. In some embodiments, this includesmeasuring deviations in the conductivity parameter over a number of nsamples and then summing them together. When the sum of n exceeds thethresholds, it trigger a warning and/or shut down the system. In somecases, this can be a magnitude difference of difference (e.g., an errorvalue of 100 for lower amp processes and an error value of 150 for 300amp processes). Whereas, normal operating conditions can tend togenerate only single digit error values. For example, the conductivityparameter can be measured at an electrical connection between the plasmaarc and the pulse width modulation control system 508. This exampleelectrical connection is marked in FIG. 5 as current measurement point510.

Then, the specified conductivity parameter set point can be compared tothe detected conductivity parameter of plasma arc current. In someexamples, a difference between the set point conductivity parameter andthe actual detected conductivity is calculated. In some embodiments, thedifference can be used to calculate an error term signal (e.g., alsoreferred to as a deviation) that indicates how much the actualconductivity parameter deviates from its intended value. In some cases,this determines errors or fluctuations in plasma arc current. The errorterm signal can include a compilation of multiple comparisons of thespecified current set point to the detected plasma arc current atmultiple times during a material processing operation/over a givenperiod of time. The multiple times can be performed over a predeterminedtime interval. The multiple comparisons can be a rolling sum of multipleerror term signals detected over consecutive plasma arc currentmeasurements. In some cases, the multiple error term signals can beabout 5 error term signals to about 20 error term signals.

The error term signal can be compared to a threshold amount. Forexample, the system can reference a lookup table of example error termsignal threshold amounts. In some examples, the threshold value can bepredefined for a particular cutting process or material. That is, thethreshold can vary between different types of workpiece materials and/orthicknesses, different cutting speeds, or different cutting operations(e.g., fine cutting, rough cutting, gouging, etc.). Based on adetermination that the error term signal exceeds the threshold amount, aplasma arc shut down sequence can be initiated to extinguish the plasmaarc to limit damage to the plasma arc torch body. That is, as discussedabove, an increasing error term can indicate or suggest a physicaldegradation of the consumable in the plasma torch. Therefore, failingconsumables in a torch body, which can cause fluid to expel within thetorch body, can create unwanted noise in the plasma arc electricalsignals, which results in the error term unintentionally increasing. Theplasma shut down sequence can be implemented to shut down the torchbefore incurring further consumable failure which could damage the torchbody. In some embodiments, the plasma shut down sequence can includedecreasing a plenum gas pressure so that the plasma arc is extinguishedin a lower pressure environment. In this aspect, the system typicallyneed not shut down the torch in a controlled manner to protect theconsumable for future use as with the ramp-down aspects described abovedue to the degradation of the consumables. That is, the torch protectionmethods here are typically implemented to determine that a consumable ora set of consumables are failing, and thus are beyond a point from whichfurther use is possible. Thus, the plasma shut down sequence can includean abrupt, immediate shut down of the torch. In some embodiments, theshut down of the torch can include decreasing the plasma arc current andshutting down the pump. In some embodiments, the shut down of the pumpcan include forcing a system reset and replacement of the consumables.That is, since the system has determined that the consumables havefailed or are about to fail, the system can go into a lock mode in whichthe system is essentially inoperable until the failed consumables arereplaced. In some cases, this limits or prevents a user from attemptingto further use the torch with the failing consumables, which can lead todamaging the torch. This can be useful to protect the torch by limitingor preventing a total electrode blow out.

Other example methods are possible to implement one or more of the abovetorch protection features. For example, some methods of operating aplasma arc torch can include initiating a plasma arc in the plasma arctorch and sending a signal to operate the plasma arc at a definedoperating current and detecting an actual current level of the plasmaarc. As described above, the current level of the plasma arc can bemeasured, for example, at an electrical connection between the plasmaarc and the pulse width modulation control system 508. This exampleelectrical connection is marked in FIG. 5 as current measurement point510.

Next a deviation between the defined operating current and the actualcurrent level can be determined and, in response to determining that thedeviation is below a threshold error level, permitting the plasma torchto continue operating. That is, the system can be configured tosubstantially continuously monitor the plasma arc current on a rollingbasis and, as long as the measured current is within particulardeviation range of the current set point, permit the system to operate.The current can be detected and redetected after one or more samplecycle time periods, and the updated deviation between the definedoperating current and the redetected actual current level can bedetermined. For example, the plasma arc current can be measured andcompared for a second, third, fourth, etc. time. In some cases, thedeviations can include a rolling sum of the previous 5 to 20measurements (e.g., about ten measurements). In some embodiments, theactual current can be compared to the set current. The differencebetween the actual and set current can be integrated over a time periodand compared to a threshold value. In some examples, this comparison canbe performed every 330 microseconds.

However, once it is determined that the updated deviation meets orexceeds the threshold error level, a plasma arc extinguishing sequencecan be initiated to extinguish the plasma arc to limit damage to theplasma torch body. As discussed above, the comparison of the deviationsto a threshold error levels can include referencing a lookup table ofthreshold values that correspond to one or more material processingscenarios. The plasma arc extinguishing sequence can be implemented toquickly shut down the plasma torch and, in some cases, lock the torchfrom further use until the damaged consumables are replaced.

In some aspects, the temperature of liquid coolant through the torch canbe used to predict consumable failure, either alone or in combinationwith the failure prediction described above that uses plasma arccurrent. For example, in some embodiments, fluctuations (e.g., rises) incoolant temperatures (e.g., electrode coolant temperature) can besuggestive of an impending failure event (e.g., a prediction of an endof life event). Therefore, if the coolant temperature can be monitoredfor such fluctuations, the end of life event can be predicted. In somecases, use of coolant temperature can be used as a general warning thata failure may be approaching. This can be viewed as a “yellow” cautionlight to put the user on notice. Whereas the current monitoring abovecan be more of a determinative notice that a failure is impending. Thiscan be viewed as a “red” stop light in which the power supply can takeaction to stop the torch operation. This can be useful to provide anindividualized end of life prediction for each electrode used in atorch, and typically does not require consideration of cutting errors,marking starts, ramp-down errors, cutting processes, etc. These methodsalso help to limit or prevent electrode blowout. In addition toprotecting an electrode, the methods can also reduce torch damage andwarranty returns, as well as reduce scrap and workpiece failures.

Embodiments of this aspect can include any of various features. Forexample, the system can use temperature probes to monitor coolant flowto and/or from an electrode during torch operation. The system caninclude two temperature probes to monitor, for example, two differenttime scenarios (e.g., an on and an off temperature) or two places (e.g.,flow temperatures before and after the electrode). In some embodiments,the temperature rise across the probes can be monitored over a period oftime to generate a temperature profile, which can be used to predict acatastrophic failure. Additionally, the power input to the electrode canbe monitored. Spikes or significant fluctuations (e.g., significantrises) in the electrode temperature can indicate upcoming failure. Forexample, in some cases, the temperature profile can be compared to athreshold profile to determine an upcoming failure.

Unless otherwise stated, various features and embodiments of thesemethods can be combined with one another or with the other aspectsdescribed herein.

Material Workpiece Piercing Detection Methods

In addition to or in alternative to the consumable and torch protectionmethods described above, the electrical and gas delivery systemsdescribed above can be used during a startup sequence of a plasma arctorch. Specifically, the electrical systems described herein to predict(e.g., using the power supply itself) when a workpiece piercing sequencehas completed so that the torch can be moved and a material processingoperation can begin. By enabling pierce detection, various processingparameters can be adjusted to increase plasma torch performance, life,and efficiency.

For example, in some embodiments, the PWM monitoring and/or arc voltageestimation discussed above can be used to detect when the plasma arc hassuccessfully pierced the workpiece and to begin motion of the plasmatorch before arc voltage becomes unsustainable. In some embodiments, arcvoltage estimation can be used to lower the total arc voltage requiredduring piercing by providing insights to altering the process parameters(such as, plasma pressure, arc current), or by altering the pierce gastype (e.g., inert gas such as, argon, nitrogen, etc. in the shield gas).Once the pierce has been detected using the PWM monitoring and/or arcvoltage estimation, the process parameters can be adjusted to a cuttingprocess (e.g., as opposed to those desired for a pierce process). Forexample, the arc voltage could be lowered after piercing (breakthrough)is detected.

This can be particularly useful because, as depicted in FIG. 11, thereis typically a tradeoff between standoff distance (e.g., the distancebetween the torch and the workpiece) and arc voltage as illustrated bypierce traces taken at different torch to workpiece/arc attachment pointdistances (e.g., 1 inch, 1.3 inches, and 1.6 inches). For 2 inchpiercing, there is typically enough voltage available to break throughthe plate, even at a standoff height of 1.6 inches. One issue with thisobservation is that the torch typically must start motion within 0.5seconds to 1 second of breaking through the workpiece. With a singlepierce time, and the fact that the time to get through will typicallyvary with plate composition and consumable wear, the end user mayexperience undesired or failed start up attempts, such as misfires(pop-outs) or failures to pierce. Note that the voltage when the arcpierces the workpiece is relatively low compared to the maximum voltagesseen shortly after breakthrough. The time available for the end user toinitiate table motion after breakthrough but before arc extinction ismuch shorter when using larger standoffs.

Additionally or alternatively, in some cases, parameters, such as gasflows (e.g., shield gas or plasma gas flows) or electrical parameters(e.g., current or voltage) can be adjusted based on the plasma piercingto increase (e.g., maximize) the usable life of the consumable and/or toincrease the maximum pierce capacity of the system. For example, themaximum pierce rating of a mechanized system is typically fundamentallylimited by either damage to the shield or the maximum voltage availableto sustain the arc. These limits are often in conflict. For example, ahigh torch to work distance can help protect the shield from damage butrequires a high (e.g., very high) voltage to keep the arc fromextinguishing. In some embodiments, the systems and methods describedherein can be used to enable high torch to work distances with lowervoltage requirements. For example, certain gases (e.g., non-oxidativeplasma or shield gases (e.g., inert gases such as argon gases)) can beselectively delivered to make workpiece piercing easier by requiringless plasma arc voltage. As a result of the reduced plasma arc voltagerequired to pierce a workpiece, a consumable may be used for morepiercing operations than could otherwise be accomplished withconventional gases. Additionally, the reduced voltage required can beuseful to increase standoff height during piercing. In some embodiments,the flow of argon gases can be stopped once piercing is detected, forexample, using observed changes in the monitored parameters of theelectrical system (e.g., in the pulse width modulation system).

Referring to FIG. 9, electrical parameters can be monitored during aworkpiece piercing operation and based on changes to the observedparameters, a plasma system can detect when piercing has been completed.FIG. 9 illustrates three different example observations for differentworkpiece thicknesses (e.g., ½ inch, ¾ inch, and 1 inch thicknesses). Asdepicted, the thicker workpieces typically require greater time to fullypierce through, as reflected by spikes (e.g., rapid increases) in therate of change of plasma arc voltage. Based on these observations anddiscoveries, various methods can be carried out for initiating a plasmaarc, piercing a workpiece, and performing/transitioning to a cuttingoperation. Specifically, the time when the arc pierces the workpiece canbe detected in a plasma arc signal, such as a pulse width modulationparameter (or rate of change thereof) or a voltage parameter (e.g., rateof voltage change over time (dV/dt)), one or more threshold values, orsome combination of both.

For example, in some aspects, methods of starting (e.g., initiating) aplasma arc with a plasma arc torch to pierce a workpiece and detectingthe plasma piercing through the workpiece to begin a cutting sequencecan include measuring an electrical characteristic of an electricalsignal associated with a plasma arc between an electrode of a plasma arctorch and a workpiece to be processed. In some embodiments, theelectrical characteristic can include a pulse width modulationcharacteristic (e.g., duty cycle) or a rate of change of the pulse widthmodulation characteristic of a signal being sent to generate andmaintain the plasma arc. In some cases, the characteristic can include avoltage-based characteristic, such as a rate of change of the voltage.For example, with regards to the embodiments described above andillustrated in FIGS. 4 and 5, a pulse width modulation control system508 can be used to set a desired current set and, based on the piercingprocess, the characteristic (e.g., pulse width modulation duty cycle)can vary as the piercing occurs. In some examples, the system can beoperated at 15 kHz, which is a time period of 66 microseconds. Asdiscussed above, the pulse width modulation duty cycle is the percent“on” time.

Next, the characteristic can be monitored during operation of the torchover a time period of a workpiece piercing sequence. That is, a plasmaarc torch power supply can measure and monitor the characteristic duringpiercing to determine how the characteristic varies (e.g., how the pulsewidth modulation duty cycle varies) in reaction to piercing through theworkpiece.

The characteristic (e.g., the monitored pulse width modulation dutycycle) can then be compared to a threshold value. In some cases, thethreshold value can be a predetermined value that suggests that theplasma has pierced (e.g., broken through) the workpiece. That is, themonitored pulse width modulation duty cycle can increase during piercingand particularly spike (quickly increase) when the plasma breaks throughthe workpiece. The threshold can vary based on various limitations, suchas the desired cutting process (e.g., fine cutting, gouging, etc.),desired cutting speed, material to be processed, etc. Thus, a plasmasystem can be programmed with multiple threshold values, where eachcorresponds to a particular cutting process. In some cases, thethreshold values can be determined empirically. In some cases, thethreshold values can be stored in a lookup table.

In response to, or otherwise based on, determining that a measuredcharacteristic meets and/or exceeds the threshold value, the plasmasystem (e.g., the power supply) can begin the cutting sequence and causethe plasma arc torch to move relative to the workpiece to form the cut.That is, when the measured characteristic meets or exceeds the thresholdvalue, the system is effectively determining that the plasma has piercedthe workpiece. Once piercing is detected, the power supply can initiatemovement of the torch to form a cut along its path of motion. In someexamples, this can include sending a signal, from the power supply to agantry controller (e.g., a CNC controller), to cause relative motionbetween the torch and the workpiece.

Detecting piercing and initiating motion in this manner can bebeneficial compared to some conventional systems that are based simplyon piercing time in that the power supply can actively track thepiercing and begin the cutting motion specifically on the piercecompletion. In some cases, this can result in faster cutting processes.In some cases, there can be a delay between detecting piercing andbeginning the cutting process. For example, when piercing is firstdetected, it is possible that the plasma has just broken though theworkpiece and the hole therethrough is semi-conical or parabolic andholding the piercing plasma in place for some period of time can help tomake the hole more substantially cylindrical. In effect, the plasma canbe held in place to substantially ream the hole formed by plasma beforebeginning cutting. In some cases, the time delay can be about 0milliseconds to about 500 milliseconds.

The ability to detect workpiece piercing using the electrical systemsdescribed herein can help reduce or eliminate several issues ordrawbacks with conventional workpiece piercing and cutting procedures.For example, conventional techniques typically rely on predeterminedpiercing time periods before moving to a cutting process (e.g., movingthe torch to form a cut). For example, empirical data can be used todetermine an estimated time needed to pierce. In some cases, theestimated time can be an estimated worst case scenario for the timerequired for an electrode with 95% of its usable life gone to fullypierce a workpiece. By way of example, this time period could be about50 microseconds. However, similar, but new, consumables may only needabout 10 microseconds. Thus, these worst case, predetermined timeperiods have the potential to be far longer than actually needed topierce the workpiece. That is, in some cases, a plasma can pierce theworkpiece and remain in place for an unnecessarily long time beforebeing moved to form a cut. As a result of the excess time in place, theelectrode can undergo unnecessary wear, time can be wasted (e.g.,because the torch could instead be moved), as well as damage to theworkpiece caused by the workpiece being overly worked or heated by thestationary plasma. Therefore, by detecting when the plasma actuallybreaks through the workpiece, the power supply can take action to beginthe cutting sequence, for example, by changing gas flows or beginningtorch motion.

In some cases, the gases (e.g., shield or plasma gases) can be changedfrom a piercing set of gases, to a cutting set of gases. For example,during the piercing sequence, a gas mixture can be delivered having aninert (e.g., argon or nitrogen) gas as a plasma gas and/or a shield gasto surround the plasma. Upon completion of piercing, the gases can bechanged to a cutting gas configuration.

Other methods are possible. For example, in some aspects, methods ofperforming a piercing sequence to pierce a hole in a workpiece with aplasma arc torch can include initiating an arc between the electrode anda nozzle of the plasma arc torch during a pilot arc mode. Next, the arccan be transferred from the corresponding nozzle to a workpiece forpiercing and cutting the workpiece. With the arc transferred, the plasmasystem (e.g., power supply) can begin a workpiece piercing sequence. Forexample, in some cases, the piercing sequence can include increasingplasma arc current to increase the energy being directed to theworkpiece. Alternatively or additionally, the piercing sequence caninclude increasing the gas pressure, for example, to help remove moltenmaterial from the workpiece pierced hole.

During the piercing sequence, a characteristic associated with a pulsewidth modulated signal being provided to the torch to maintain plasmaarc between the torch and the workpiece can be measured. As discussedabove, electrical system parameters, such as the pulse width modulationduty cycle, or rate of change thereof, as well as changes in plasma arcvoltage can suggest physical information during piercing. Specifically,fast increases or bursts in the characteristic associated with a pulsewidth modulated signal being provided to the torch can suggest that theplasma has pierced (e.g., broken through) the workpiece. Thus, duringthe piercing sequence, the characteristic can be compared to apredetermined value (e.g., the threshold value). And in response todetermining that a measured characteristic meets or exceeds thepredetermined value, relative movement can be initiated between theworkpiece and the plasma arc torch to begin a cutting sequence to form acut along the workpiece. For example, the relative movement between theworkpiece and the plasma arc torch can be initiated by sending a signalfrom a power supply in communication with the plasma arc torch to amovement controller mechanically coupled to the plasma arc torch. Asdescribed above, in some embodiments, the initiated relative movementbetween the workpiece and the plasma arc torch is delayed by apredetermined time period. The delay can be useful to open asemi-conical pierced hole in the workpiece into a substantiallycylindrical pierced hole using plasma expelled from the plasma arctorch.

Unless otherwise stated, various features and embodiments of thesemethods can be combined with one another or with the other aspectsdescribed herein.

Inert Gas Assisted Workpiece Piercing Methods

Alone or in combination with the various other aspects described above,other techniques can be implemented during piercing to increase theusable life of an electrode (e.g., the number of piercing sequences forwhich an electrode can be used). As discussed herein, these methods canbe implemented in association with conventional piercing sequences(e.g., using predetermined piercing times discussed above) or with oneor more of the more controlled pierce detections sequences herein (e.g.,using voltage estimation or pulse width modulation). In some cases, thecombination of methods has the advantage of reducing pierce times, sincehigher voltage pierces often have shorter pierce times, and could alsohelp in the interaction with the CNC control. Specifically, performing apierce sequence (e.g., after a plasma arc has been transferred to aworkpiece and a pilot hole is being formed in the workpiece beforemoving the torch to form a cut) in the presence of a non-oxidative gas,such as an inert gas (e.g., an argon gas or a nitrogen gas), can help tocreate an environment in which the voltage needed to complete the pierceis reduced. As a result of the lower voltage for piercing, a plasma arcelectrode can be used for a greater number of piercing operations. Forexample, the inert gas (e.g., argon or nitrogen gas) and lower voltagescan permit the torch from being further away from the workpiece duringpiercing (e.g., have a higher standoff height), which can also helpincrease the usable life. In some cases, the greater standoff height canhelp reduce molten workpiece material spatter from adhering to the torch(e.g., consumables). These changes could be detrimental to the cuttingperformance and would only be used while piercing. Therefore, piercedetection by arc voltage estimation, PWM analysis, and/or processing,and/or change in the gas type and settings (shield or plasma) can beselectively used during piercing and switched to the optimum setting forcutting operation. In some embodiments certain parameters could be usedduring piercing and certain parameters could be used during cutting.Examples could be changing the arc current, plasma gas pressure, orshield pressure. For example, FIG. 10 depicts experiments in which theuse of argon gas can result in lower plasma arc voltage during piercing.

Based on these observations and discoveries, various methods can becarried out for initiating a plasma arc, piercing a workpiece, andperforming a cutting operation. For example, in some aspects, a methodfor increasing a usable number of pierce operations for an electrode ofa plasma arc torch can first include initiating an arc between theelectrode and a corresponding nozzle of the plasma arc torch during apilot arc mode and transferring the arc from the corresponding nozzle toa workpiece for piercing and cutting the workpiece. Once attached to theworkpiece, a current of the plasma arc can be increased. The piercingcan also include delivering a gas mixture comprising an argon gas as aplasma gas or a shield gas to surround the arc. In some embodiments, thegas mixture can include the argon gas being delivered from a meteringconsole located on or near the plasma arc torch as described above. Thepresence of the inert gas (e.g., argon, nitrogen, etc.) can reduce amaximum plasma arc voltage required to break through the workpieceduring the piercing sequence. Additionally, in some cases, the presenceof the inert gas can actually increase the time to penetrate and piercethe workpiece.

Then, in response to determining that the piercing sequence hascompleted, the plasma system can begin a cutting sequence and moving theplasma arc torch relative to the workpiece. Determining that thepiercing sequence has completed can be accomplished in any of variousways including using predetermined time periods and cut charts, as wellas the various pierce detection methods described above. In someembodiments, the beginning of a cutting sequence can include providing amotion signal to a CNC controller to initiate relative motion betweenthe torch and the workpiece. In some embodiments, the beginning of acutting sequence can include delivering a second gas mixture to surroundthe arc that is distinct from gas mixture delivered during the piercingsequence. That is, in some examples, a set of gases can be used duringpiercing and then changed for cutting. For example, the gas mixturedelivered during the piercing sequence can include a plasma gascomprising argon and a shield gas comprising oxygen, and the beginning acutting sequence can include delivering a second gas mixture to surroundthe arc comprising a plasma gas comprising nitrogen and a shield gascomprising argon.

Additionally or alternatively, in some aspects, methods of operating aplasma arc torch can include operating the plasma arc torch in an arcinitiation mode by generating an arc between an electrode and a nozzlein the presence of a first set of shield and plasma gases. The first setof gases can include pilot arc mode gases. In some cases, the pilot arcmode gases can be similar or the same as subsequently used piercinggases.

The methods can next include operating the plasma arc torch in atransferred piercing mode by transferring the arc from the nozzle to aworkpiece in the presence of a second set of shield and/or plasma gases.At least one of the shield or plasma gases of the second set at leastpartially includes an argon gas.

Based on determining that the arc has fully pierced the workpiece, theplasma arc torch can be operated in a transferred cutting mode. Forexample, the transferred cutting mode can include moving the torchrelative to the workpiece in the presence of a third set of shield andplasma gases. In some embodiments, the operating the plasma arc torch ina transferred cutting mode can include providing a motion signal to aCNC controller to initiate the moving of the torch. In some embodiments,the third set of shield and plasma gases can be distinct from the secondset. That is, a certain set of gases (e.g., having argon) can be usedfor piercing and then a different set of gases can be used for cutting.

Unless otherwise stated, various features and embodiments of thesemethods can be combined with one another or with the other aspectsdescribed herein.

While various aspects and embodiments have been described herein, itshould be understood that they have been presented and described by wayof example only, and do not limit the claims presented herewith to anyparticular configurations or structural components. Thus, the breadthand scope of a preferred embodiment should not be limited by any of theabove-described exemplary structures or embodiments, but should bedefined only in accordance with the following claims and theirequivalents. Therefore, other embodiments fall within the scope of thefollowing claims.

What is claimed:
 1. A method for preserving a usable life of a plasmaarc electrode consumable installed in a plasma arc torch, the methodcomprising: measuring, with a control system, a pulse width modulationcharacteristic of an electrical signal being provided to the torch togenerate a plasma arc between the torch and a workpiece to be processed,wherein the pulse width modulation characteristic comprises at least oneof a pulse width modulation value, a rate of change of the pulse widthmodulation value, or a pulse width modulation duty cycle; determining,with the control system, a pulse width modulation threshold valuecorresponding to the pulse width modulation characteristic based on aplurality of cutting operation parameters, wherein the plurality ofcutting operation parameters comprises at least one consisting from thegroup of workpiece material type, workpiece material thickness, cuttingspeed, or cutting operation; monitoring, with the control system, thepulse width modulation characteristic during operation of the torch overa time period; comparing, with the control system, the pulse widthmodulation characteristic to the pulse width modulation threshold value;and responsive to determining that a measured pulse width modulationcharacteristic meets and/or exceeds the pulse width modulation thresholdvalue, initiating an arc extinguishing sequence to preserve the life ofthe electrode.
 2. The method of claim 1 wherein the pulse widthmodulation threshold value is greater than about 80%.
 3. The method ofclaim 1 further comprising comparing a bus voltage to an incomingvoltage to determine a duty cycle percentage of at least one of achopper or a pulse width modulator.
 4. The method of claim 1 wherein thearc extinguishing sequence comprises causing a decrease in a plenum gaspressure.
 5. The method of claim 1 wherein the arc extinguishingsequence comprises causing a decrease in arc current.
 6. The method ofclaim 1 wherein the arc extinguishing sequence is completed in less thanabout 70 milliseconds.
 7. The method of claim 1 wherein the measuredpulse width modulation characteristic meeting or exceeding the pulsewidth modulation threshold value indicates an increasing distancebetween the plasma arc torch and an arc attachment point where theplasma arc attaches to the workpiece.
 8. The method of claim 7 whereinthe increasing distance indicates that the torch has reached at leastone of an edge of the workpiece.
 9. The method of claim 7 wherein theincreasing distance indicates that the torch has reached a kerf area.10. The method of claim 1 wherein the comparing the pulse widthmodulation characteristic to the pulse width modulation threshold valuecomprises referencing a lookup table of pulse width modulation thresholdvalues for a particular cutting process.
 11. A method of operating aplasma arc torch and extinguishing a plasma arc during a materialprocessing operation to limit electrode wear resulting from unintendedarc loss, the method comprising: initiating the plasma arc between anelectrode of the plasma arc torch and a workpiece to be processed withthe torch; translating the torch along the workpiece and performing aprocessing operation to the workpiece; advancing the torch towards avoid defined by the workpiece; responsive to the torch reaching thevoid, detecting, with a control system, a lengthening of the plasma arcbetween the arc attachment point and the torch, wherein the detecting ofthe lengthening of the plasma arc comprises: monitoring, by the controlsystem, a characteristic of an electrical signal being provided to thetorch to generate the plasma arc; comparing, by the control system, thecharacteristic to a threshold value; and based on the detected plasmaarc lengthening, initiating an arc extinguishing sequence to limit wearof the electrode, wherein the arc extinguishing sequence comprises atleast one consisting from the group of: causing a decrease in a plenumgas pressure, or causing a decrease in arc current.
 12. The method ofclaim 11 wherein the void comprises a peripheral edge of the workpiece.13. The method of claim 11 wherein the characteristic comprisesmeasuring a bus voltage.
 14. The method of claim 13 further comprisingcomparing the bus voltage to an incoming voltage to determine a dutycycle percentage of at least one of a chopper or a pulse widthmodulator.
 15. The method of claim 11 wherein the characteristiccomprises a pulse width modulation value.
 16. The method of claim 11wherein the characteristic comprises a rate of change of a pulse widthmodulation value.
 17. The method of claim 11 wherein the arcextinguishing sequence is completed in less than about 70 milliseconds.18. A plasma arc material processing system comprising: a torchincluding a torch body configured to receive a set of consumablecomponents connected to the torch body and configured to generate aplasma arc; and a power supply operably connected to the torch, thepower supply comprising: a means for measuring a pulse width modulationcharacteristic of an electrical signal being provided to the torch togenerate the plasma arc between the torch and a workpiece to beprocessed, wherein the pulse width modulation characteristic comprisesat least one consisting from the group of a pulse width modulationvalue, a rate of change of the pulse width modulation value, or a pulsewidth modulation duty cycle; a means for determining a pulse widthmodulation threshold value corresponding to the pulse width modulationcharacteristic based on a plurality of cutting operation parameters,wherein the plurality of cutting operation parameters comprises at leastone consisting from the group of workpiece material type, workpiecematerial thickness, cutting speed, or cutting operation; a means formonitoring the pulse width modulation characteristic during operation ofthe torch over a time period; a means for comparing the pulse widthmodulation characteristic to the pulse width modulation threshold value;and a means for, responsive to determining that a measured pulse widthmodulation characteristic meets and/or exceeds the pulse widthmodulation threshold value, initiating an arc extinguishing sequence topreserve the life of the electrode.